1. Field of the Invention
The present invention generally relates to an electronic component having terminal caps at its opposite ends and, more particularly, to an improvement in the terminal caps for electric connection with an external circuit.
2. Background Art
For the purpose of discussion of the prior art pertinent to the present invention, reference is made to FIGS. 1 and 2 of the accompanying drawings which illustrate different types of ceramic capacitors in longitudinal sectional representations, respectively.
Referring to FIG. 1, the ceramic capacitor shown therein and generally identified by 20 is of a leadless type and comprises an open-ended, generally tubular body 23 made of dielectric material, and a terminal cap 22 mounted on each of the opposite ends of the tubular body 23. The tubular dielectric body 23 has its inner and outer peripheral sufaces deposited with inner and outer electrode layers 24 and 25. The inner electrode layer 24 on the inner peripheral surface of the tubular dielectric body 23 extends from near one of the opposite ends of the tubular dielectric body 23 and over the annular end face of the other of the opposite ends of the same tubular dielectric body 23, terminating on the outer peripheral surface of the tubular dielectric body 23 at a position spaced a slight distance axially inwardly from such annular end face of the other of the opposite ends of the tubular dielectric body 23 so that, when and so long as the corresponding terminal cap 22 is mounted on such other of the opposite ends of the tubular dielectric body 23, the inner electrode layer 24 can be electrically connected with such corresponding terminal cap 22. On the other hand, the outer electrode layer 25 on the outer peripheral surface of the tubular dielectric body 23 extends from one of the opposite ends of the tubular dielectric body 23 towards a position spaced a distance axially inwardly from the other of the opposite ends of the tubular dielectric body 23 so that, when and so long as the corresponding terminal cap 22 is mounted on such one of the opposite ends of the tubular dielectric body 23, the outer electrode layer 24 can be electrically connected with such corresponding terminal cap 22. In any event, each of the terminal caps 22 has a solder layer of substantially uniform thickness electroplated all over the entire surface thereof.
The connection between each of the terminal caps 22 and the associated electrode layer 24 or 25 is carried out in the following manner according to the prior art. After the terminal caps 22 have been mounted on the respective ends of the tubular dielectric body 23 subsequent to the formation of the inner and outer electrode layers 24 and 25, the tubular dielectric body 23 with terminal caps 22 thereon is heated to permit that portion of the solder layer which is deposited on an inner surface area 22a of each of the terminal caps 22 to melt so that the melted solder, when subsequently solidified, can connect the respective terminal cap 22 electrically and physically with the associated electrode layer 24 or 25. In this way, not only are the terminal caps 22 connected electrically with the inner and outer electrode layers 24 and 25, but also the terminal caps 22 are firmly mounted on the tubular dielectric body 23.
The tubular dielectric body 23 with the terminal caps 22 soldered respectively to the inner and outer electrode layers 24 and 25 is then subjected to a sheathing process during which first a sheathing material in a fluidized state is applied through a transfer applicator roll, supported for rotation in one direction in a vertical plane and partially immersed in a vessel containing the fluidized sheathing material, for example, synthetic resin, to the tubular dielectric body 23, then tumbling over the outer periphery of the applicator roll being rotated, thereby forming a generally cylindrical protective coating 26 to complete the leadless ceramic capacitor 20.
Shown by 30 in FIG. 2 is the ceramic capacitor of axial lead type, that is, the ceramic capacitor having a pair of axially extending lead wires 31 extending outwardly from the respective terminal caps 22. The axial lead type shown in FIG. 2 is substantially identical in structure with the leadless ceramic capacitor 20 of FIG. 1 except for the lead wires 31. Also however, the outer protective coating 26 forming a sheath on the ceramic capacitor 30 substantially completely encases the ceramic capacitor 30 except for the lead wires 31, which are permitted to protrude outwards for electric connection with the external circuit.
The soldering of each terminal cap 22 hitherto carried out in the manufacture of any one of the ceramic capacitors 20 and 30 has the following problems. Specifically, where the thickness of the solder layer formed on each of the terminal caps 22 is small, the molten solder will not be uniformly distributed between the respective terminal cap 22 and the tubular dielectric body 23 because the amount of the molten solder on the inner surface area 22a of the respective terminal cap 22 is small. Once this happens, one or both of the terminal caps 22 will not be firmly connected with the respective electrode layer 23 or 24, failing to establish a proper electric connection therebetween. Thus, the reliability of the electric and physical connection between the terminal cap and the associated electrode layers on the tubular dielectric body is reduced.
On the other hand, where the thickness of the solder layer formed on each of the terminal caps 22 is great, the above discussed reliability can be increased because an ample amount of solder exists inside the terminal caps 22. However, the solder deposited on the outer surface area 22b of each of the terminal caps 22 will, when melted and then solidified, form a sagging drop of solder material as indicated by 22c in FIG. 1. The presence of the solder sagging 22c distorts the shape of the resultant ceramic capacitor with the roundness reduced consequently. Because of this, when the ceramic capacitor is subjected to the previously discussed sheathing process, it will fail to rotate smoothly in unison with the rotating applicator roll and, therefore, no smooth and uniform coating of the protective coating 26 can be accomplished.
The irregular shape of the ceramic capacitor resulting from the uneven coating of the sheathing material particularly causes a further problem when the ceramic capacitor is used in connection with an automatic component mounting apparatus operable to automatically mount the electronic components onto printed circuit boards. More specifically, the presence of the sagging 22c in the ceramic capacitor at one or both of the terminal caps 22 allows such ceramic capacitor to be caught somewhere in the automatic component mounting apparatus with the consequence that the ceramic capacitor fails to be properly mounted onto a printed circuit board. In addition, the uneven coating of the sheathing material may bring about reduction in the sealing of the ceramic capacitor which in turn reduces the moisture-proofness of the ceramic capacitor.
On the other hand, when the ceramic capacitor of axial lead type is to be mounted on a printed circuit board with the use of the automatic component mounting apparatus, the lead wires 31 are inserted through associated holes in the printed circuit board and then soldered to printed conductors on the printed circuit board. During the soldering of the lead wires 31 to the printed conductors, heat used for this purpose tends to be transmitted to the terminal caps 22 through the lead wires 31 with the solder layer on the outer surface areas 22b of the respective terminal caps 22 being again melted consequently. When the solder layers are so melted again, and in the event that the thickness of the solder layer on each of the terminal caps is great, the solder tends to undergo a thermal expansion to such an extent as to result in the blow-off of a portion of the solder in the form of a tear-drop from a gap, indicated by 32 in FIG. 2, between the terminal cap 22 and the protective coating 26. Once this occur, the drop of solder so blown off from the ceramic capacitor may constitute a cause of undesirable shortcircuiting.